First issue - when simplemail gets e-mails from imap they all marks unread regardless their state in server. Other email clients (e.g. evolution, thunderbird) set right state of each email.
Second issue. Sent emails don't falls into "sent" imap folder on server.
Third issue. I create imap/smtp account, get all mail. After restart firefox folder "inbox" is empty. On server all emails still exist. I tried delete/recreate account, delete/reinstall extension, delete extension folder ~/.mozilla/firefox/profinename/simplemail. On pressing "refresh" button simplemail don't show any errors but can't fetch any mail.

As recap this extension is unuseable for me, search for alternative email client.